Composition, bath, and process for chromium plating



May 26, 1953 Filed NOV. '7, 1950 F/G'URE J. E. STARECK 2,640,022

COMPOSITION, BATH, AND PROCESS FOR CHROMIUM PLATING 3 Sheets-Sheet l Q i v INVENTOR 7/ "NO/JVaM/JJVOJ f JESSE E. STARECK a f 4. BY ATTO NEYS May 26, 1953 Filed Nov. '7, 1950 J. E. STARECK COMPOSITION, BATH, AND PROCESS FOR CHROMIUM PLATING 5 Sheets-Sheet 2 JESSE E. STARECK May 26, 1953 J. E. STARECK 2,640,022

COMPOSITION, BATH, AND PROCESS FOR CHROMIUM PLATING Filed Nov. 7, 1950 3 Sheets-Sheet 3 2 CURVE A= N0. Ql

/5 5/ INVENTOR JESSE E. STARECK CURVE B= K- 9/! Patented May 26, 1953 COMPOSITION, BATH, AND PROCESS FOR CHROMIUM PLATING Jesse E. S tareck, Birmingham, Mich assignor to United Chromium, Incorporated, New York, N. Y., a corporation of Delaware Application November 7, 1950, Serial No. 194,502

24 Claims. 1

This invention relates to chromium plating baths, compositions for making up and maintaining such baths, and to a chromium plating process.

The essentia1 feature of my invention is the utilization of what I shall call suppression agents to maintain and regulate the catalyst acid radical content of a chromium plating bath of the chromic acid type. I give below several examples of compositions to be used in making up chromic acid baths and these compositions include suppression agents which have the effect of controlling the content of catalyst acid radicals in the bath from whatever source and controlling the solubility of the salts used to introduce such radicals into the bath. For instance, in Example I the suppression agents are potassium bichromate and strontium chromate.

The present invention provides a bath and process having numerous advantages, among which are:

1. Self-regulation of the catalyst acid radical content at or near the optimum concentration in baths of any desired chromic acid content;

2. Reduced or eliminated need for analytical control;

3. The ability to take up appreciable amounts of catalyst acid radical introduced as impurities or as drag-in Without exceeding the operating limits in the bath for such radicals;

4-. Superior covering power;

5. High operating emciency such as low power consumption, minimum rejections and low operating costs;

6. Wide bright-plate range;

7. High plating speed;

8. Good cathode activating properties;

9. Bright plate direct on stainless steel;

10. Avoidance of iridescent film on articles only partly covered in chromium plating.

The bath and composition consist essentially of chromic acid (CrOs) a sulfate radical bearing salt having a limited solubility in the chromic acid bath, a non-catalytic compound of the cation associated with the sulfate radical bearing salt, a silicofluoride bearing salt having a limited solubility in the chromic acid bath, and a non-catalytic compound of the cation associated with the silicofluoride salt. For instance, by adding the composition given in Example I below to water a chromic acid bath is produced which contains strontium sulfate and potassium silicofluoride, each in excess of its solubility in the bath, and also strontium chromate and potassium bichromate in solution.

In my invention, the total amount of catalyst acid radicals in solution is furnished from a plurality of substances, each supplying a portion of the total catalyst acid radical, the amount of each radical used being determined by the solubility of the particular salt used in introducing that acid radical into the bath. Thus, when the catalyst acid radical from the salt is deplenished in the bath for one reason or another, the acid adical lost from the solution is replenished by acid radical from the catalyst acid radical bearing salt remaining in the bath as undissolved residue. The solubility of each of the catalyst acid radical bearing salt in the bath is in turn further regulated by the addition to the bath of other compounds having no catalyst acid radical but containing a cation common to that of the salts containing the catalyst acid radicals. The catalyst acid radicals are the sulfate and the silicofluoride radicals.

Examples of useful non-catalytic strontium and potassium compounds are SrCOs, Sr(OI-I)2, SrCrO4, etc., and KOH, K2CO3, K2CI'O4, KzCrzOv, etc.

The control of the solubility of the catalyst acid radical salts in the bath is desirable in that selfregulation of the catalyst acid radical content of the bath at or near the optimum concentration may thus be achieved at any desired chromic acid concentration, with the advantages peculiar to these different concentrations.

Examples of compositions of matter formulated according to the present invention in percentages are:

Example I Chromic acid '70 to Potassium bichromate 5.5 to 15.5 Strontium chromate 0.6 to 3.6 Potassium silicofluoride 2.5 to 7.5 Strontium sulfate 1.4 to 3.4

Example II Chromic acid 85 to Potassium bichromate 2.5 to 7.5 Strontium chromate 0.5 to 1.5 Potassium silicofluoride 1.3 to 4.1 Strontium sulfate 0.7 to 1.9

Example III Chromic acid 18 to 72.8 Sodium bichromate 23.4 to '70 Strontium chromate 0.8 to 3 Sodium silicofluoride 1.8 to 6 Strontium sulfate 1.2 to 3 Example IV Chromic acid 59 to 86.4 Sodium bichromate 11.6 to 35 Strontium chromate 0.5 to 1.5 Sodium silicofluoride 1.0 to 3.0 Strontium sulfate 0.5 to 1.5

Example V Chromic acid 85 to 95 Sodium bichromate 1.6 to 5 Strontium chromate 1.1 to 3.3 Sodium silicofluoride 1.9 to 5.5 Strontium sulfate 0.4 to 1.2

Example VI Chromic acid 62.5 to 87.5 Potassium bichromate 5.7 to 17.7 Strontium chromate 4.3 to 12.3 Potassium silicofluoride 2.0 to 6.0 Strontium sulfate 0.5 to 1.5

Example VII Chromic acid 85 to 95 Potassium bichromate 2.4 to 6.6 Strontium chromate 1.5 to 4.5 Potassium silicofluoride -e 0.9 to 2.5 Strontium sulfate 0.4 to 1.2

Example VIII Chromic acid 85 to 95 Sodium bichromate 2.3 to 6.7 Strontium chromate 0.6 to 1.8 Potassium silicofiuoride 1.5 to 4.5 Strontium sulfate 0.6 to 2.0

Example IX Chromic acid 79 to 95 Sodium bichromate 2.3 to 11.7 Sodium silicofluoride 1.5 to 5.1 Strontium sulfate 1.2 to 4.2

Compositions given in Example I above are adapted to give satisfactory solutions over a range of chromic acid concentration from about 150 g./l. to about 400 g./1., the optimum varying somewhat with the basis metal upon which the chromium is plated and also with the type of application of chromium. For example, when chromium plating stainless steel, a chromic acid concentration of about 150 g./l. to about 200 g./l. is usually preferred. When plating over a nickel plate, a chromic acid concentration of about 200 g./l. to about 250 g./l. is usually preferred. For the production of a chromium deposit with maximum leveling action, a chromic acid concentration from about 250 g./l. to about 350 g./l. is usually preferred.

The composition of Example II above is adapted to give satisfactory solutions over a range of chromic acid concentrations from about 250 g./l. to about 500 g./1. The chief use of the composition given in Example II is as a maintenance composition for baths made up from Example I, but it may also be used to make a plating bath. When used as a plating bath for chromium plating over nickel plate, the optimum chromic acid range is from about 2'75 g./l. to about 350 g./1. When used as a plating bath for producing heavy chromium deposits with maximum leveling action, a concentration range of about 350 g./l. to about 500 g./l. is usually preferred. The higher chromic acid concentrations made possible by Example II result in a bath having having high conductivity with minimum treeing of deposits for heavy chromium plating.

Example III represents a composition corresponding to that in Example I except that sodium salts are used in place of potassium salts and all that has been said with respect to Example I applies to Example III.

Similarly, the composition of Example IV is the same as that of Example II with the exception that the sodium salts are used in place of potassium salts of Example II and What has been said with respect to Example II and the baths made therefrom applies to Example IV.

The composition of Example V is especially useful for barrel chromium plating. It may be used over a range of chromic acid concentration from about 300 g./l. to about 500 g./l., the optimum being about 400 g./l. These baths may also be used for ordinary decorative and for heavy chromium plating.

The compositions of Example VI above give satisfactory solutions over a range of chromic acid concentration from about to about 250 g./l., with an optimum of about g./l. The provision of a satisfactory bath having this low chromic acid concentration is one of the desirable features of my invention and gives the particular advantages of low cost and high efiiciency.

The chief use of the composition given in Example VII is as a maintenance for baths made up from Example VI. However, it is also adapted to give satisfactory plating solutions over a range of chromic acid concentrations from about 200 g./l. to about 400 g./l.

Example VIII provides a composition which is useful in making chromium plating solutions over a range of chromic acid concentrations from about 250 g./l. to about 500 g./l., with an optimum of about 400 g./l. The main feature of this composition is that it is adapted to regulate the sulfate catalyst acid radical in particular by the use of a single suppression agent such as strontium chromate as distinguished from the previous examples where both the sulfate and the silicofluoride radicals are regulated by a plurality of suppression agents. In this example only the strontium chromate is a suppression agent, sup pressing the sulfate content; the sodium bichromate does not function as a suppression agent inasmuch as the silicofluoride is present as the potassium salt, not the sodium salt, and the sodium from the sodium bichromate is present in insufficient quantity to have a suppressing effect on the silicofluoride; i. e., any sodium silicofiuoride present would be in amount less than its solubility in this solution. The sodium bichromate merely supplements the chromic acid as a supplier of chromate. This composition can be used for decorative, barrel, and heavy chromium plating.

Example IX is similar to Example VIII except that in Example IX the silicofluoride radical in particular is controlled by the use of a single suppression agent such as sodium bichromate. This composition can be used for decorative, barrel, and heavy chromium plating.

As pointed out above, the suppression agents used in the composition of Example I are strontium chromate and potassium bichromate. These same agents are used in Examples II, VI, and VII. The suppression agents used in Examples III, IV, and V are sodium bichromate and strontium chromate. The supression agent used in Example VIII is strontium chromate and that used in Example IX is sodium bichromate. The strontium in the suppression agents in all of the examples given herein suppresses directly the sulfate content in the bath and the sodium or potassium of the suppression agents suppresses directly the silicofiuoride content in the bath.

For those examples containing a non-catalytic potassium compound, it may be noted that the potassium content of such compound ranges from 0.64 to 4.7% by weight, while the strontium content of the non-catalytic strontium compound ranges from 0.21 to 5.3% by weight. In those examples having a non-catalytic sodium compound, the sodium content of such compound varies from 0.28 to 12.3% by weight, and the strontium content of the non-catalytic strontium compound varies from 0.21 to 1.4% by weight.

A special advantage of using a suppression agent for the silicofiuoride radical is not only to adjust its concentration in solution to the desired value, but also to make it less affected by temperature changes and by changes in chromic acid concentration of the bath.

Certain of the superior features of the baths made up from the compositions given above (and from similar compositions) are illustrated by the curves attached hereto and made a part hereof.

In Fig. 1, curves A, B, and C illustrate the effect on the sulfate radical concentration of baths made up using the composition of Example I to give 200 -g./l. chromic acid as CIOs, 250 g./l. chromic acid as CrOa, and 300 g./l. chromic acid as CI'Os, respectively, by the addition (i. e. dragin) of sulfate radical in amounts up to a little over 3 g./l. Curves A-l, B-l, and C-l illustrate the effect of the addition of sulfate radical (i. e. drag-in) on the sulfate radical concentration of ordinary solutions made up of only chromic acid and sulfate and with chromic acid and sulfate concentrations corresponding to those of the baths represented by curves A, B, and C, respectively. In solutions represented by curves A-l, B-i, and C-l, the sulfate radical concentration increases by the same amount as the sulfate acid radical drag-in. Thus for 1 g./l. of S04 drag-in, the S04 concentration as shown by the curve C-l, for example, is increased by 1 g./l. to a total value of about 1.83 g./l.; and likewise a 1 g./l. S04 drag-in affects the S04 concentration of solutions represented by curves A-l and B-I in the same manner and to the same extent.

By reference to curve C, it will be seen that a drag-in of l g./l. S04 will raise the S04 concentration of the bath represented by that curve from about 0.83 to about 1.0, or by 0.17 g./l. Thus the rise of the S04 concentration in consequence of the drag-in of the 1 g./l. S04 results in only about 20% increase of S04 concentration with a bath represented by curve C as-against 120% increase of S04 concentration in a solution represented by curve C-l. The comparison is similar between curves A and A-l and between curves B and B-I.

In Fig. 2, curve B illustrates the regulating effect of the supression agents in baths of 200 g./l. and 250 g./l. chromic acid concentrations made with a composition of Example III, with respect to temperature variations. Curve A represents a bath made with a composition like that of Example III but without the suppression agents. Thus from curve B of Fig. 2, at 250 g./l. of chromic acid, for a temperature variation from 90 to 130 F. the SiFs content in solution varies only from about 1.4 to 2.7 (a difference of 1.3 g./l.), whereas from curve A it will be seen that for a like change in temperature the SiFe varies from 9.8 to 13.9 (a difference of 4.1 g./l.). Again, from curve B of Fig. 2, at 200 g./l. chromic acid,

the SiFo content varies only from 1.7 to 3.4 (a difference of 1.7 g./l.). As may be appreciated, for all of the baths contemplated by the invention, theSiFs content varies little with temperature changes. Similarly, the S04 content also varies little with temperature changes, the magnitude of the variation being even less than in the case of the 811%.

Fig. 3 illustrates the effect of the suppression agents on the acid radical contents in solution. Curve A shows the effect of the sodium content from the suppression agent on the silicofluoride content. Curve B shows the effect of the Potassium from the suppression agent on the silicofiuoride content, and curve C shows the effect of strontium from the suppression agent on the sulfate content. The curves of Fig. 3 represent the effects described above in a bath having a chromic acid concentration of 250 g./l. at F. Thus, for example, as shown on curve C, if to the bath containing 8 g./l. of excess strontium (this is the strontium from the suppression agent) which as shown by curve C contains 0.5 g./l. of sulfate, there is added an amount of sulfate (as from sulfuric acid, a common drag-in material) equivalent to 4 -g./1. of strontium (i. e., 4A g./l. of sulfate), in the range of 8 to 4 g./l. of strontium on curve C, the amount of sulfate which will go into solution will be the difference between the sulfate concentrations at 8 g./1. strontium and at e g./l. strontium, i. e., 0.9 g./l. minus 0.5 g./l., or only 0.4 g./1. sulfate. Thus, only 0A g./l. sulfate goes into solution for a total addition of 4.4. g./l. sulfate, the balance being precipitated as strontium sulfate. As will be apparent, the drag-in effect has been effectively minimized. Similarly, from curves A and B it will be clearly understood that a similar relationship exists between the amount of SiFe added and the amount that goes into solution.

In this connection, and speaking in a general way, it may be observed from Fig. 3 that the concentration of sulfate radical may be depressed by addition of strontium radical to a concentration lower than the unsuppressed saturation concentration but above zero concentration, the unsuppressed saturation value being given on curve C at the point zero Sr, which point represents the sulfate content of the solution saturated with strontium sulfate in the absence of any other strontium or sulfate compound. In other words, sulfate radical concentration may be depressed anywhere in the range of 2.1 g./1. to 0.2 g./l., the latter value being taken at the right hand limit of the curve and being equivalent to about one-tenth of the unsuppressed saturation concentration. Of course, this value may be even lower if the curve is extended to the right. Similarly, the silicofluoride radical (see curve B) may be depressed by addition of potassium radical to a lower concentration but above zero, the concentration extending to as low as about onefourth, or less, of the unsuppressed saturation concentration, and it may be depressed by sodium radical (curve A) to a concentration as low as about one-fourth, or less, of the unsuppressed saturation concentration. These observations are generally applicable to the invention.

It is preferred that the completed compositions as given in Examples I to IX, inclusive, be prepared and used, but obviously the advantages of my invention can be secured equally well by preparing the composition of Example I, for instance, omitting the chromic acid, the other ingredient-s of this example being added to a-- chromic acid solution prepared independently. Similarly, the benefits of my invention may be secured by other combinations of the ingredients given in the various examples where one or more of the ingredients is added to the bath independently of the others. Also, as pointed out earlier, suppressing agents other than those specifically listed in the examples may be used equally well.

While I have given the compositions in the examples herein in terms of the percentages, it is understood of course that these may be expressed as in parts which may be readily calculated from the values given in the respective examples. Also, in view of the examples, wherein a portion oi the total CrOa content is supplied by (3103 added as such and a portion is supplied by non-catalytic salts like sodium bichromate, strontium chromate, etc., the recitation in the claims of 0103 is, unless otherwise specified, intended to cover both the C103 added per se to a composition or bath and also the 0103 added as part of a non-catalytic compound. The ClOs content may thus be seen to range from a low of about 72% in Example III to a high of about 97% in Example VII.

What is claimed is:

1. A composition 01' matter for making up an aqueous chromic acid bath for chromium plating in the range of 100 to 500 g./l. of CIOs, comprising essentially about 72 to 97 per cent by weight of CI'Oz, an amount of strontium sulfate suilicient to saturate said bath and to provide an undissolved residue of strontium sulfate in the bath, an amount of potassium silicofluoride suflicient to saturate said bath and to provide an undissolved residue of potassium silicofluoride in the bath, a soluble non-catalytic strontium compound in an amount containing 0.21 to 5.3 per cent by weight of strontium, and a soluble noncatalytic potassium compound in an amount containing 0.64 to 4.7 per cent by weight potassium.

2. A composition of matter for making up an aqueous chromic acid bath for chromium plating in the range of 250 to 500 g./l. of C1103. com prising essentially 85 to 95 per cent by weight of CrOz, sodium bichromate 2.3 to 6.7 per cent by weight, an amount of strontium sulfate which provides an undissolved residue of strontium sulfate in the bath, an amount of potassium silicofiuoride which provides an undissolved residue of potassium silicofiuoride in the bath, and a soluble non-catalytic strontium compound in an amount containing 0.26 to 0.78 per cent by weight strontium.

3. A composition of matter for making up an aqueous chromic acid bath for chromium plating in the range of 250 to 500 g./l. of CrOa, comprising essentially 79 to 95 per cent by weight of ClOs, an amount of strontium sulfate which provides an undissolved residue of strontium sulfate in the bath, an amount of sodium silicofluoride which provides an undissolved residue of sodium silicofluoride in the bath, and a soluble non-catalytic sodium compound in an amount containing 0.4 to 2.0 per cent by Weight sodium.

4. A composition of matter for making up and maintaining an aqueous chromic acid bath for chromium plating in the range of 100 to 500 grams per liter of CrOc, said composition comprising essentially about 72 to 97% by weight of C'rOs, two catalyst-supplying compounds, and a soluble non-catalytic compound, one of said catalyst-supplying compounds being strontium sulfate in an amount suiiicient to saturate said bath and to provide an undissolved residue of strontium sulfate in the bath, the other of said catalyst-supplying compounds being an alkali metal silicofluoride in an amount sufllcient to saturate said bath and to provide an undissolved residue of alkali metal silicofiuoride in the bath, said alkali metal being selected from the class consisting of potassium and sodium, said noncatalytic compound being present in an amount sufficient to suppress the concentration of one of said catalyst-supplying compounds in solution in said bath from the unsuppressed saturation concentration of the latter compound to a lower but greater than zero concentration, said noncatalytic compound being selected from the class consisting of a strontium compound and. an alkali metal compound, and the alkali metal of said alkali metal compound being the same as the alkali metal'of said alkali metal silicofluoride.

5. A composition of matter for making up and maintaining an aqueous chromic acid bath for chromium plating in the range of to 500 grams per liter of CrOa, said composition comprising essentially about 72 to 97% by weight of Cl'O3, two catalyst-supplying compounds, and a soluble non-catalytic compound, one of said catalyst-supplying compounds being strontium sulfate in an amount sufiicient to saturate said bath and to provide an undissolved residue of strontium sulfate in the bath, the other of said catalystsupplying compounds being an alkali metal silicofluoride in an amount sufficient to saturate said bath and to provide an undissolved residue of alkali metal silicofluoride in the bath, said alkali metal being selected from. the class consisting of potassium and sodium, said non-catalytic compound being a strontium compound in an amount sufficient to suppress the concentration of said strontium sulfate in solution in said bath from the unsuppressed saturation concentration of the latter compound to a lower but greater than zero concentration.

6. A composition of matter for making up and maintaining an aqueous chromic acid bath for chromium plating in the range of 100 to 500 grams per liter of ClOs, said composition comprising essentially about 72 to 97% by weight of CrOs, two catalyst-supplying compounds, and a soluble non-catalytic compound, one of said catalyst-supplying compounds being strontium sulfate in an amount sufiicient to saturate said bath and to [provide an undissolved residue of strontium sulfate in the bath, the other of said catalystsupplying compounds being an alkali metal silicofluoride in an amount suflicient to saturate said bath and to provide an undissolved residue of alkali metal silicofiuoride in the bath, said alkali metal being selected from the class consisting of potassium and sodium, said non-catalytic compound being an alkali metal compound in an amount suilicient to suppress the concentration of said alkali metal silicofluoride in solution in said bath from the unsuppressed saturation concentration of the latter compound to a lower but greater than zero concentration, and the alkali metal of said alkali metal compound being the same as the alkali metal of said alkali metal silicofluoride.

7. A composition of matter for making up and maintaining an aqueous chromic acid bath for chromium plating in the range of 100 to 500 g./l. CrOz, said composition comprising essentially about 72 to 97% by weight of C103, and the balance of the composition consisting essentially of the following four compounds: (1) strontium sulfate in an amount sufiicient to saturate said bath and to provide an undissolved residue of strontium sulfate in the bath, (2) an alkali metal silicofluoride in an amount sufficient to saturate said bath and to provide an undissolved residue of alkali metal silicofluoride in the bath, said alkali metal being selected from the class consisting of potassium and sodium, (3) a soluble noncatalytic strontium compound in an amount sufficient to suppress the concentration of the strontium sulfate in solution in said bath from the unsuppressed saturation concentration of the latter to a lower but greater than zero concentration, and (4) a soluble non-catalytic alkali metal compound in an amount suflicient to suppress the concentration of the alkali metal silicofluoride in solution in said bath from the unsuppressed saturation concentration of the latter to a lower but greater than zero concentration, and the alkali metal of said non-catalytic compound being the same as the alkali metal of said alkali metal silicofluoride,

8. A composition of matter for making up and maintaining an aqueous chromic acid plating bath for chromium plating in the range of 100 to 500 g./l. CrOs, said composition comprising essentially about 72 to 97% by weight of C103 and the balance comprising the following four compounds: (1) strontium sulfate in an amount sufficient to saturate said bath and to provide an undissolved residue of strontium sulfate in the bath, (2) an alkali metal silicofiuoride in an amount sufficient to saturate said bath and to provide an undissolved residue of alkali metal silioofluoride in the bath, said alkali metal being selected from the class consisting of potassium and sodium, (3) a soluble non-catalytic strontium compound in an amount sufficient to suppress the concentration'of the strontium sulfate in solution in said bath from the unsuppressed saturation concentration of the latter to a lower concentration, said lower concentration being in the range extending from said unsuppressed saturation concentration to about one-tenth of the unsuppressed saturation concentration, and l) a soluble non-catalytic alkali metal compound in an amount suflicient to suppress the concentration of the alkali metal silicofluoride in solution in said bath from the unsuppressed saturation concentration of the latter to a lower concentration in the range extending from said lastmentioned unsuppressed saturation concentration to about one-fourth thereof, and the alkali metal of said non-catalytic compound being the same as the alkali metal of said alkali metal silicofiuoride.

9. A composition of matter for making up and maintaining an aqueous chromic acid bath for chromium plating in the range of 100 to 500 g./l. f CrOa, said composition comprising essentially about '72 to 97% by weight of CIO3, an amount of strontium sulfate sufficient to saturate said bath and to provide an undissolved residue of strontium sulfate in the bath, an amount of sodium silicofluoride sufiicient to saturate said bath and to provide an undissolved residue of sodium silicofluoride in the bath, a soluble non-catalytic strontium compound in an amount equivalent to 0.21 to 1.4 per cent by weight of strontium, and a soluble non-catalytic sodium compound in an amount equivalent to 0.28 to 12.3 per cent by weight of sodium.

10. A composition of matter for making up and maintaining an aqueous chromic acid bath for chromium plating in the range of 100 to 500 g./l.

10 of ClOs, consisting essentially of 90 to 70% by weight of CI'Os per se, 1.4 to 3.4% by weight of strontium sulfate, 2.5 to 7.5% by weight of potassium silicofluoride, 0.6 to 3.6% by weight of strontium chromate, and 5.5 to 15.5 by weight of potassium bichromate.

11. A composition of matter for making up and maintaining an aqueous chromic acid bath for chromium plating in the range of 100 to 500 grams per liter of CrOs, consisting essentially of 95 to 85% by weight of CI'Os per se, 0.? to 1.9% by weight of strontium sulfate, 1.3 to 4.1% by weight of potassium silicofluoride, 0.5 to 1.5% by weight of strontium chromate, and 2.5 to 7.5% by weight of potassium bichromate.

12. A composition of matter for making up and maintaining an aqueous chromic acid bath for sulfate in an amount sufiicient to saturate said.

bath and to provide an undissolved residue of strontium sulfate in the bath when said composition is added to said bath in the proportion of at least 5 parts of the composition to parts of the chromic acid in said bath, the other of said catalyst-supplying compounds being an alkali metal silicofiuoride in an amount sufficient to saturate said bath and to provide an undissolved residue of alkali metal silicofluoride in the bath when said composition is added to said bath in the proportion of at least 5 parts of the composition to 95 parts of the chromic acid in said bath, said alkali metal being selected from the class consisting' of potassium and sodium, said non-catalytic compound being present in an amount sufficient to suppress the concentration of one of said catalyst-supplying compounds in solution in said bath from the unsuppressed saturation concentration of the latter compound to a lower but greater than zero concentration, said non-catalytic compound being selected from the class consisting of a strontium compound and an alkali metal compound, and the alkali metal of said alkali metal compound being the same as the alkali metal of said alkali metal silicofluoride.

13. A composition of matter for making up and maintaining an aqueous chromic acid bath for chromium plating in the range of to 500 grams per liter of 0103, said composition comprising essentially the following four compounds: (1) strontium sulfate in an amount sufiicient to saturate said bath and to provide an undissolved residue of strontium sulfate in the bath when said composition is added to said bath in the proportion of at least 5 parts of the composition to 95 parts of the chromic acid in said bath, (2) an alkali metal silicofluoride in an amount sufficient to saturate said bath and to provide an undissolved residue of alkali metal silicofluoride in the bath when said composition is added to said bath in the proportion of at least 5 parts of the composition to 95 parts of the chromic acid in said bath, said alkali metal being selected from the class consisting of potassium and sodium, (3) a soluble non-catalytic strontium compound in an amount sufficient to suppress the concentration 'of the strontium sulfate in solution in said bath 1'1 fluoride in solution in said bath from the unsuppressed saturation concentration of the latter to a lower but greater than zero concentration, and the alkali metal of said non-catalytic compound being the same as the alkali metal of said alkali metal silicofluoride,

14. A composition for use in making up and maintaining an aqueous chromic acid bath for chromium plating in the range of 100 to 500 grams per liter of ClOa, said composition comprising essentially about 2.4 to about 17.7 parts by Weight of potassium bichromate, about 0.5 to about 12.3 parts by weight of strontium chromate, about 0.9 to about 7.5 parts by weight of potassium silicofluoride, and about 0.4 to about 3.4 parts by weight of strontium sulfate.

15. A composition for use in making up and maintaining an aqueous chromic acid bath for chromium plating in the range of 100 to 500 grams per liter of CrOs, said composition comprising essentially about 1.6 to about '70 parts by weight of an alkali metal bichromate, about 0.5

.to about 12.3 parts by weight of strontium chromate, about 0.9 to about 7.5 parts by weight of an alkali metal silicofluoride, about 0.4 to about 3.4 parts by weight of strontium sulfate, said alkali metal being selected from the class consisting of potassium and sodium.

16. A composition for use in making up and maintaining an aqueous chromic acid bath for chromium plating in the range of 100 to 500 grams per liter of CrOz, said composition comprising about 1.6 to about 17.7 parts by weight of an alkali metal bichromate, about 0.9 to about 7.5 parts by weight of an alkali metal silicofiuoride, about 0.4 to about 4.2 parts by Weight of strontium sulfate, said alkali metal being selected from the class consisting of potassium and sodium.

17. A composition for use in making up and maintaining an aqueous chromic acid bath for chromium plating in the range of 100 to 500 grams per liter of ClO3, said composition comprising essentially about 1.6 to about 70 parts by weight of sodium bichromate, about 0.5 to about 3.3 parts by weight of strontium chromate, about 1.0 to about 6 parts by weight of sodium silicofluoride, and about 0.4 to about 3 parts by Weight of strontium sulfate.

18. A composition for use in making up and maintaining an aqueous chromic acid bath for chromium plating in the range of 100 to 500 grams per liter of CrOa, said composition comprising essentially about 0.6 to about 1.8 parts by weight of strontium chroma'te, about 1.5 to about 4.5 parts by weight of potassium silicofluoride, and about 0.6 to about 2.0 parts by weight of strontium sulfate.

19. An aqueous bath for chromium plating comprising essentially 100 to 500 g./l. of CrOs, two catalyst-supplying compounds, and a soluble non-catalytic compound, one of said catalystsupplying compounds being strontium sulfate in an amount sufficient to saturate said bath and to provide an undissolved residue of strontium sulfate in the bath, the other of said catalyst-supplying compounds being an alkali metal silicofiuoride in an amount suflicient to saturate said bath and to provide an undissolved residue of alkali metal silicofluoride in the bath, said alkali metal being selected from the class consisting of potassium and sodium, said non-catalytic compound being present in an amount sufficient to suppress the concentration of one of said catalystsupplying compounds in solution in said bath from the unsuppressed saturation concentration of the latter compound to a lower but greater than zero concentration, said non-catalytic compound being selected from the class consistin of a strontium compound and an alkali metal compound, and the alkali metal of said alkali metal compound being the same as the alkali metal of said alkali metal silicofluoride.

20. An aqueous bath for chromium plating comprising essentially to 500 g./l. of CrO'i, strontium sulfate in an amount sulficient to saturate said bath and to provide an undissolved residue of strontium sulfate in the bath, an alkali metal silicofluoride in an amount sufficient to saturate said bath and to provide an undissolved residue of alkali metal silicofluoride in the bath, said alkali metal being selected from the class consisting of potassium and sodium, a soluble non-catalytic strontium compound in an amount sufiicient to suppress the concentration of the strontium sulfate in solution in said bath from the unsuppressed saturation concentration of the latter to a lower but greater than zero concentration, and a soluble non-catalytic alkali metal compound in an amount suificient to suppress the concentration of the alkali metal silicofluoride in solution in said bath from the unsuppressed saturation concentration of the latter to a lower but greater than zero concentration, and the alkali metal of said non-catalytic compound being the same as the alkali metal of said alkali metal silicofluoride.

21. An aqueous bath for chromium plating comprising essentially 100 to 500 g./l. of 010:, strontium sulfate in an amount sufficient to saturate said bath and to provide an undissolved residue of strontium sulfate in the bath, an alkali metal silicofluoride in an amount sufficient to saturate said bath and to provide an undissolved residue of alkali metal silicofluoride in the bath, said alkali metal being selected from the class consisting of potassium and sodium, a soluble non-catalytic strontium compound in an amount sufilcient to suppress the concentration of the strontium sulfate in solution in said bath from the unsuppressed saturation concentration of the latter to a lower concentration, said lower concentration being in the range extending from said unsuppressed saturation concentration to about one-tenth of the unsuppressed saturation concentration, and a soluble non-catalytic alkali metal compound in an amount sufficient to suppress the concentration of the alkali metal silicofluoride in solution in said bath from the unsuppressed saturation concentration of the latter to a lower concentration in the range extending from said last-mentioned unsuppressed satura tion concentration to about onefourth thereof, and the alkali metal of said non-catalytic compound being the same as the alkali metal of said alkali metal silicoiluoride.

22. A process of electrodepositing chromium comprising essentially passing current from an anode to an article-cathode immersed in an aqueous bath comprising essentially 100 to 500 g./l. 0103, two catalyst-supplying compounds, and a soluble non-catalytic compound, one of said catalyst-supplying compounds being strontium sulfate in an amount sufficient to saturate said bath and to provide an undissolved residue of strontium sulfate in the bath, the other of said catalyst-supplying compounds being an alkali metal silicofluoride in an amount sulficient to saturate said bath and to provide an undissolved residue of alkali metal silicofluoride 13 in the bath, said alkali metal being selected from the class consisting of potassium and sodium, said non-catalytic compound being present in an amount suiiicient to suppress the concentration of one of said catalyst-supplying compounds in solution in said bath from the unsuppressed saturation concentration of the latter compound to a lower but greater than zero concentration, said non-catalytic compound being selected from the class consisting of a strontium compound and an alkali metal compound, and the alkali metal of said alkali metal compound being the same as the alkali metal of said alkali metal silicofiuoride.

23. A process of electrodepositing chromium comprising essentially passing current from an anode to an article-cathode immersed in an aqueous bath comprising essentially 100 to 500 g./l. CrOa, strontium sulfate in an amount sulficient to saturate said bath and to provide an undissolved residue of strontium sulfate in the bath, an alkali metal silicofiuoride in an amount sufiicient to saturate said bath and to provide an undissolved residue of alkali metal silicofiuoride in the bath, said alkali, metal being selected from the class consisting of potassium and sodium, a soluble non-catalytic strontium compound in an amount sufficient to suppress the concentration of the strontium sulfate in solution in said bath from the unsuppressed saturation concentration of the latter to a lower but greater than zero concentration, and a soluble non-catalytic alkali metal compound in an amount sufficient to suppress the concentration of the alkali metal silicofluoride in solution in said bath from the unsuppressed saturation concentration of the latter to a lower but greater than zero concentration, and the alkali metal of said non-catalytic compound being the same as the alkali metal of said alkali metal silicofiuoride.

24. A process of electrodepositing chromium comprising essentially passing current from an anode to an article-cathode immersed in an aqueous bath comprising essentially to 500 g./l. CIO3, strontium sulfate in an amount sufficient to saturate said bath and to provide an undissolved residue of strontium sulfate in the bath, an alkali metal silicofiuoride in an amount sufiicient to saturate said bath and to provide an undissolved residue of alkali metal silicofluoride in the bath, said alkali metal being selected from the class consisting of potassium and sodium, a soluble non-catalytic strontium compound in an amount sufficient to suppress the concentration of the strontium sulfate in solution in said bath from the unsuppressed saturation concentration of the latter to a lower concentration, said lower concentration being in the range extending from said unsuppressed saturation concentration to about one-tenth of the unsuppressed saturation concentration, and a soluble non-catalytic alkali metal compound in an amount sufiicient to suppress the concentration of the alkali metal silicofiuoride in solution in said bath from the unsuppressed saturation concentration of the latter to a lower concentration in the range extending from said last-mentioned unsuppressed saturation concentration to about one-fourth thereof, and the alkali metal of said non-catalytic compound being the same as the alkali metal of said alkali metal silicofiuoride.

JESSE E. STARECK.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,857,547 Humphries May 10, 1932 2,040,881 Rojas May 19, 1936 2,050,478 Wickenhiser Aug. 11, 1936 2,095,995 Mardick Oct. 19, 1937 

1. A COMPOSITION OF MATTER FOR MAKING UP AN AQUEOUS CHROMIC ACID BATH FOR CHROMIUM PLATING IN THE RANGE OF 100 TO 500 G./1. OF CRO3, COMPRISING ESSENTIALLY ABOUT 72 TO 97 PER CENT BY WEIGHT OF CRO3, AN AMOUNT OF STRONTIUM SULFATE SUFFICIENT TO SATURATE SAID BATH AND TO PROVIDE AN UNDISSOLVED RESIDUE OF STRONTIUM SULFATE IN THE BATH, AN AMOUNT OF POTASSIUM SILICOFLUORIDE SUFFICIENT TO SATURATE SAID BATH AND TO PROVIDE AN UNDISSOLVED RESIDUE OF POTASSIUM SILICOFLUORIDE IN THE BATH, A SOLUBLE NON-CATALYTIC STRONTIUM COMPOUND IN AN AMOUNT CONTAINING 0.21 TO 5.3 PER CENT BY WEIGHT OF STRONTIUM, AND A SOLUBLE NONCATALYTIC POTASSIUM COMPOUND IN AN AMOUNT CONTAINING 0.64 TO 4.7 PER CENT BY WEIGHT POTASSIUM. 